Piezoelectric temperature compensating circuit



June 16, 1942.

PIEZOELECTRIC TEMPERATURE COMPENSATING CIRCUIT P. ODELL Original FiledAug. 2, 1940 ll Q13 9, an i 16 I T A T J L 1 i f i if H J F i0 za )7 if]24 23 22 g I z1 T J T INVENTOQ 'HALCOLN P C DHL' ATTOQNEV generator ofthe Rochelle salt type.

Patented June 16, 1942 oF ic Y mazosmo'rmc TEMPERATURE com ransa'rmocmcm'r Malcolm P. Odell, Cleveland, Ohio, assign'or to The BrushDevelopment Company, Cleveland, Ohio, a corporation of Ohio Originalapplication August 2, 1940, Serial No. 349,738. Divided and thisapplication October 3, 1941, Serial No. 413,496

11 Claims.

This invention relates in, general to the re.- ductlon of the effects oftemperature on the characteristics of energy transforming systems'whichembody piezoelectric devices (as disclosed in my co-pending applicationSerial No. 349,'l38, filed August 2, 1940, of which this application isa division), and it relates more particularly to automatic means forreducing the effect of temperature on the sensitivity of systems whichembody piezoelectric transducers.

The principal object of this invention is to provide automatic means forreducing the effect of temperature on the sensitivity of a system whichembodies a temperature-sensitive piezoelectric Other objects andfeatures of the invention will be apparent from the followingspecification and accompanying drawing in which several embodiments aredescribed.

Fig. 1 is a schematic wiring diagram of a transmission path which isconnected to a piezoelectric motor unit, the diagram illustrating onemode in which the invention may be applied to an amplifier transmissionpath which transmits signal to the motor unit.

Fig. 2 is a schematic wiring diagram of a transmission path which isconnected to a piezoelectric generating unit, the diagram illustratingone mode in which the invention may be applied to the amplifiertransmission path which receives signal from the generator unit.

Fig. 3 is a schematic wiring diagram of a piezoelectric generatingdevice which has an amplifier system coupled thereto and illustrates theway in which a modification of the invention may be applied to theamplifier system.

Within recent years, Rochelle salt piezoelectric crystal units have comeinto general use in various types of electro-acoustical andelectro-mechanical devices such as microphones, earphones, phonographpickups, loud-speakers, and vibration pickups. Such units offer manyadvantages such as simplicity, low cost, and uniform sensitivity for awide range of frequencies. It has been found, however, that when thesedevices are introduced into many of the circuit arrangements which aregenerally used, they frequently exhibit an undesirabletemperature-induced variation in sensitivity. When a piezoelectricgenerator device utilizing a Rochelle salt crystal unit is connected toa circuit whose input impedance is high in comparison with the impedanceof the crystal unit itself throughout the useful frequency range, theterminal voltage does not vary appreciably with changes in temperature.Likewise, there may be little variation when a motor unit is suppliedwith signal from a high impedance source, as explained hereafter.However, the impedance of a Rochelle salt crystal unit used in either amotor or a generator device increases as the temperature rises above orfalls somewhat below about 23 C. Consequently, the

voltage sensitivity of a generator device that is connected to a lowimpedance circuit decreases with such temperature changes. Likewise,when a piezoelectric motor device is driven by a signal derived from aconstant voltage source, such as an ordinary triode amplifier circuithaving a low internal output impedance, the amplitude of motiondecreases as the temperature increases above about 23 C. and in mostcases also decreases as the temperature falls below this value. Whilethis latter decrease in sensitivity at the lower temperatures is causedat least partly by the increased stifiness of the mounting pads and thedamping materials that are commonly used, the efiect of temperature onthe sensitivity is frequently the predominant cause.

It will be noted that the sensitivity versus temperature variations aresubstantially the reverse of the impedance versus temperaturevariations, both in the case of a generator device feeding a lowimpedance circuit and in the case of a motor 'device driven from a lowimpedance source.

'This fact has been utilized by prior workers in above or falls somewhatbelow about 23 C.,

because there is a concurrent temperature-induced change in theimpedance match between crystal unit and amplifier. This larger voltageoiisets to some extent the loss of sensitivity. However,.such anexpedient is usually not practical because the frequency characteristicof the unit is adversely affected. Moreover, the signal handlingcapacity of the output stage of the amplifier is reduced because theload impedance presented to it by the unit decreases as the frequencyincreases. I

The effect of temperature on sensitivity can also be reduced byconnecting a small condenser in series with the crystal motor unit andsupplying the driving voltage from an amplifier having a comparativelylow internal output resistance. The voltage applied to the crystal unitthen varies .be connected in various ways.

with changes in temperature so that the sensitivity is maintainedsubstantially constant. The frequency characteristic is not affectedbecause theimpedance of both the crystal unit-and the condenser isessentially capacitive reactance. However, if this type of circuit isused, a large percentage of the available output voltage of the tosubstantially the. same temperature variations, the impedance versustemperature characteristic of the pilot unit may be employed to vary thetransmission factor of the network so as to compensate the sensitivityversus temperature variations resulting from changes in the temperatureof the primary unit. In such a system the pilot unit is included in themain transmission network as part of an attenuator. The systemaccordingly is quite simple. Furthermore, the system functionsautomatically to control the compensation.

The essential feature of this newcircuit re sides in the use of anattenuator which consists of the pilot unit with one or more fixedimpedance elements the impedance of which does not 1 vary withtemperature.

The voltage drop appearing across theoutput terminalsfof this attenuatoris applied to the succeeding amplifier stage. The impedanceelements maybe resistive, capacitive, or inductive in nature and may In mostapplications of the invention, however, a single capacitive element issuitable. In the latter instance, the attenuator takes the'form of asimple capacitive voltage divider which consists of a fixed condenser inseries with the pilot unit, and the voltage drop across the pilot unitis Consequently, neither ance without introducing instability, andtherefore it'is desirable to use 'a compensating network that providesthe necessary amount of low frequency correction for any temperature.The present invention provides such correction.

The invention is also applicable to other types of circuits and can beused in various types of amplifiers, but since the essential feature isa applied to the following section of the main;

transmission network. Although this type of compensating circuit isparticularly useful in amplifiers which feed piezoelectric motordevices, it is also applicable to amplifiers which are fed by generatingdevices. It is particularly useful in conjunction with a generatingdevice which is loaded by the capacity of its connecting cable. Amircrophone or phonograph pickup located at a considerable distance fromthe amplifier is an example of such a device.

Another important application is in, amplifiers used in conjunction withlow frequency crystal vibration pickups and other similar generatingdevices whose low frequency response is limited by the loading effect ofthe input resistance of the first amplifier stage. Since'the impedanceof piezoelectric crystalline material is predominantly capacitive, theimpedance of piezoelectric crystal units increases as the frequencydecreases. Because of this, a high input resistance is required in orderto obtain good low frequency-response. If this resistance is notsumciently high, temperature-induced variations occurring in thecapacity of the unit produce an effect on the response in the lowfrepilot piezoelectric unit contained in an attenuator network which isconnected between successive sections ofa transmission network, thedescription of a simple amplifier circuit arrangement will illustratethe invention.

The application of the invention to an amplifier feeding a Rochelle saltcrystal motor device is illustrated in the wiring diagram shown inFig. 1. A signal source I is connected to the first'amplifying tube 2,and the output of this tube is applied to the gain regulating attenuatorwhich characterizes the invention. This attenuator, or voltage divider,consists of a fixed condenser 3 in serieswith a pilot crystal unit 4,which is preferablyconstructed similar to the motor unit and located ina position adjacent to it. The voltage appearing across the pilot unitis applied to the input circuit of the final triode amplifier tube 5which'preferably has a low output impedance, and the output of this tubeis connected to the piezoelectric motor device 6. The condenser 3 of thevoltage divider also serves to prevent the positive potential on theplate of the first tube 2 from being applied to the grid of the secondtube 5. In order to prevent direct current voltage from being applied tothe motor crystal unit, a filter consisting of resistor I and condenserI in used. The use of resistor I is desirable became the leak- .ageresistance of the condenser 8 may be less than that of the crystal unit.The two amplitric generating device, such as a microphone or phonographpickup; This circuit is particularly useful when the microphone orpickup is connected to the input terminals of the amplifier through along cable whose shunt capacitive reactance is low enough to reduce theterminal voltage of the primary crystal unit when its impedance risesbecause of a change in temperature. A-piezoelectric generating device 9is connected to the input amplifier tube H by means of the connectingcable Ill. The output circuit of this first amplifier stage is connectedto the gain regulating attenuator which consists of the fixed condenserI2 and the pilot piezoelectric unit l3. The voltagedeveloped across thepilot unit is applied to the grid circuit of the second amplifier tubeIt, and the plate circuit of this tube .is connected to the outputterminals l5, it of the amplifier. The circuit arrangement of thevoltage divider and the amplifier stages is the same as that illustratedin Fig. 1. Since the pilot piezoelectric unit should preferably beexposed to substantially the same temperature as the primary unit, thisapplication of the invention may not always be useful. However, thistype of circuit can also be used with a piezoelectric generator devicethat is shunted by a condenser in order to extend the useful frequencyrange to a lower frequency. By choosing'a shunting condenser whosereactance is low in comparison with the input resistance of theamplifier for allfrequencies throughout the desired range, the inputvoltage is reduced but the low frequency response is made relativelygreater. pilot unit, due to a change in temperature, varies the gain ofthe amplifier in such a manner as to compensate for the variation ininput voltage due to the change in the impedance of the generatorcrystal unit.

Compensation for the effect of temperature is obtained in the samemanner in both of these circuits. When the temperature rises above ordecreases below approximately 23 C., the impedance of the pilot unitincreases, and so a larger percentage of the output signal voltage ofthe first amplifier tube is applied to the grid circuit of thesucceeding amplifier tube. The output of the latter tube is accordinglyincreased proportionately, so that the gain of the amplifier as a wholeis increased. This increase in the gain of The variation in theimpedance of the the amplifier compensates for the loss in 'sensitivityof the crystal motor device-in the circuit shown in Fig. 1, andcompensates for the loss in input voltage in the circuit of Fig. 2.Since the gain of the amplifier is reduced at temperatures the effect ofthe shunt resistance of the input circuit on the low frequency responseof the device at various temperatures. The two amplifier stagesillustrated in this diagram are the same as in Figs. 1 and 2, but thegain-regulating attenuator, or voltage divider, which consists of tworesistors l1, l8 connected in series with the pilot crystal I9, isdesigned to control the gain at only the lower frequencies. This isaccomplished by'applying the sum of the voltages appearing acrossresistor I8 and the pilot unit to the succeeding amplifier stage. Atmedium and high frequencies the impedance of the pilot crystal unit islow in comparison with the resistance of this arm or branch of theattenuator, and so variations in the impedance of the pilot unit due tochanges in temperature have a negligible effect on the gain of theamplifier. The coupling condenser 20 should have a large capacity sothat its reactance is small in comparison with the input impedance ofthe attenuator for all frequencies throughout the desired range. Theresistance of the grid leak 2! of the second amplifier stage should behigh in comparison with the impedance of the pilot crystal unit at alluseful frequencies, and the resistance of resistor [1 plus the effectiveinternal impedance of the first amplifier stage should be large comparedwith the resistance of resistor 18. The ratio of the resistance ofresistor l8 to the impedance of the pilot unit 19 for which thesensitivity is high, the possibility of circuits for the purpose ofintroducing temperature-compensating effects. For example, atransmission system having no amplification could embody this invention;additional amplifyi stages could be used; or the circuit could be of thepush-pull amplifier type, in which case two pilot units might bedesirable, or the circuit could be arranged to use only one. Also, theinvention may be applied to transducer devices which use crystallineRochelle salt units or which use other types of piezoelectriccrystalline material. The pilot unit may also be constructed of someother type of piezoelectric material, but throughout the workingtemperature range its impedance versus temperature characteristic musteither complement the sensitivity versus temperature characteristic ofthe transducer device, or be similar thereto. In this latter case thevoltage across the fixed impedance element of the attenuator is appliedto the succeeding amplifier stage. In view thereof, it will beunderstood that the above description is to be considered asillustrative, and the scope of the invention is to be limited only bythe following claims.

In the foregoing discussion it has been pointed out that the impedanceversus temperature characteristics of the pilot unit complement thesensitivity versus temperature characteristics of the main or primarytransducer unit. This language has been used heretofore and is used inthe following claims to mean ,that when the temperature change is suchas to decrease the sensitivity of the primary unit, the voltage dropapplied by the attenuator to the following section of the transmissionpath is increased; and conversely, when the temperature change is suchas to increase the sensitivityof the primary unit,'the voltage dropapplied by the attenuator to the following section of the transmissionpath is decreased.

Having now disclosed the invention, what I claim is:

1. A piezoelectric system comprising the combination of a transmissionpath which includes an input circuit and an output circuit, each of saidcircuits having a pair of input terminals and a pair of outputterminals; a temperature sensitive piezoelectric generator unitconnected to said acteristics of said connected piezoelectric gen-v mustbe approximately equal tothe ratio of the resistance of grid leak 22 tothe impedance of the piezoelectric generator crystal unit 23 at anygiven frequency and temperature. If these con ditions are satisfied,this voltage divider network will substantially compensate for thevariation in the input voltage applied to the first stage at lowfrequencies with changes in temperature. The output terminals 2 and 25may be connected to an indicating-device, another amplifier, or anyother suitable circuit.

It will be understood from these examples that the invention can beapplied to transmission path so as to be in shunt with the inputterminals of the input circuit; a pilot piezoelectric unit havingimpedance versus temperature characteristics which complement in somemeasure the sensitivity versus temperature charerator unit, said pilotand generator units being physically disposed relative to each other soas to be subjected at substantially the same time to substantially thesame temperature conditions; and an attenuator which combines arelativelyfixed temperature-insensitive impedance element in seriescircuit with a corrective impedance branch which includes said pilotpiezoelectric unit, said attenuator being connected to the outputterminals of the input circuit with at least the pilot, piezoelectricunit thereof in shunt with the input terminals of the output circuit.

2. A piezoelectric system as claimed in claim 1 wherein said correctiveimpedance branch consists solely'of said pilot unit.

3. A piezoelectric system as claimed in claim 1 wherein each of saidinput and output circuits includes an electronic amplifier in circuitbetween its input and. output terminals.

4. A piezoelectric system as claimed in claim 1 wherein said fixedimpedance element consists of various types of a resistor, and saidcorrective impedance branch includes a resistorin series with the saidpilot unit.

I 5. A piezoelectric system comprising the combination of: apiemelectric generator whose sensitivity varies with temperature andwhich is subjected to temperature changes when in use; and correctivetransmission means'for transmitting signal voltages from said generatorto a load. and for automatically altering the amplitude of the signalvoltage so transmitted to increase the amplitude when the temperaturechange is such as to reduce the sensitivity of the generator, andconversely to decrease theamplitude when the temperature change is suchas to increase the sensitivity of the generator, whereby the effects oftemperature induced changes in the sensitivity of the generator arecompensated at least in part, said corrective means comprising: (A) asignal voltage attenuator having its input in shunt with said signalvoltage and including a pilot piezo-.

electric unit whose impedance varies with temperature and is therebyeffective in producing temperature-induced variations in the output ofthe attenuator relative to its input-said pilot unit being disposedrelatlve to said generator so as to be subjected to substantially thesame temperature variations as the latter at substantially the sametime; and (B) means for transmitting signals from the output of saidattenuator to said load.

6. A piezoelectric system as claimed in claim 5 wherein said correctivetransmissionlmeans is a said attenuator is a coupling unit between apair multi-stage electronic amplifier, and wherein of stages of saidamplifier.

'l. The system as claimed in claim 5 wherein said signal voltageattenuator consists of said pilot unit in series circuit with asubstantially temperature-insensitive impedance, and wherein the outputof the attenuator is the signal voltage drop across said pilot unit.

8. The system as claimed in claim 5 wherein said signal voltageattenuator consists of said pilot unit in series circuit with acondenser, and wherein the output of the attenuator is the signalvoltage drop across said pilot unit.

9. An improved piezoelectric system for compensating the .effects ortemperature-induced variations in the sensitivity of a piezoelectricgenerator unit, said system comprisingthe combination of a piezoelectricgenerator unit subject to temperature-induced variations in sensitivity;a load; an attenuator having a ratio of output to input which varieswith temperature and which complements in some measure the sensitivityversus temperature characteristics exhibited by said generator unit whenin operation in said systern, said attenuator including a pilotpiezoelectric unit which is physically disposed relative to saidgenerator unit so as to be subjected to substantially the sametemperature variations as said generator unit; circuit means fortransmitting signal from said generator to the input of said attenuator;and means for transmitting the output signal of said attenuator to saidload.

10. An improved system as claimed in claim 9 wherein said attenuatorconsists of a fixed capacity in series with said pilot unit, and whereinthe output of the attenuator is the signal voltage drop across the saidpilot unit.

11. An improved system as claimed in claim 9 wherein each of saidtransmission means includes an'electronic amplifier.

MALCOLM P. ODELL.

. e cna'nm'cmn or comc'nofi. v I Patent No. 2,286,157, June 16, 191 2.

mmom v. ODELL.

It 1; herebi certified that' errer appeara in the printed 'epeclncation'of the above mmbered peteht requiring correction a: follows: Page 14;,first column, 1111535, claim strike out v" said attenuator is a couplingunit between a pair" and insert the same after "wherein" in line 31;,same claim; and that the Said Letters Patent should be read with thiscorrectien therein that the aeme may conform to the record of the cagein the rPatent. Office Y i Signed and-healed this 25th day of August, A.D. 191m.

' Henry Yen Arsdale; (Seal) Acting Commiesioner of Patents.

CERTIFICATE OF CORRECTION.

Patent No. 2,286,157, 1 June 16, 1914.2.

' HALCOLH P. ODELL.

It 1; hereby certified that error appears in the printed 'epecirication'of the mm numbered potent requiring correction as follows: Page 14;,first column, line 53, c1a1m 6, strike out said attenuator is a couplingunit between a peir" and insert the same after "wherein" in line 5 4-,same claim; and that the said Letters Patent should be read with thiscorrection therein that the some may conform to the record of the cagein the Patentofiice Signed and 'eealed this 25th o of August, A. D. 19%.

Henry Von Aredale; (Seal) Acting Commissioner of Patents.

. 'CERTIFIGA1'E or conmacnofi. I Patent No. 2,286,157.; June 16, 19h2.

mmom r1 ODELL.

It is hereby certified thaterror appears in the printed specification ofthe above numbered potent requiring correction as follows: Page 11;,first column, line 35, claim 6, strike out said attenuator is a couplingunit between a pen" and insert the same after "wherein" in line 51;,same claim; and that the said Letters Patent should be read with thiscorrection therein that the some may conform to the record of the casein the Patentofficel' I I Signed and-sealed this 5th on of August, A. D.19h2.

Henry Von Arsdale; (Seal) Acting Commissioner of Patents.

